The present invention relates to a photoelectronic device for optical communications.
Optical communications are rapidly developing as a new communication system. Optical communications use light as an information transmitting medium and optical fibers are used as paths for transmitting the light. Optical communications are also called "optical fiber communications" and can effect communications with lower loss, wider band, longer distance and higher capacity than those of communications using coaxial cables of the prior art. A semiconductor laser element is generally used as a light source and is one of the light sources, which is not only small and light weight, but is also capable of performing direct, high-speed modulation and high-capacity communication with a low-voltage drive.
For realizing long-distance communications, an optical repeater is often required, which repeater serves to amplify d.c. power which attenuates in an optical fiber. The optical repeater is also required to have another function of monitoring whether or not the frequency of the light input from the optical fiber to the optical repeater is equal to that of the light output from the optical repeater to another optical fiber. For this function, a semiconductor laser device is required for optical communications, which is equipped with a light emitting element and a light receiving element in a common package.
As a semiconductor laser device, a laser module for communications has been proposed as described in "Hitachi Review", Vol. 33, No. 4, pp 193 to 198 (1984). This semiconductor laser device is assembled into a directly opposed package, in which the leading end of an optical fiber is opposed to face the end face of an oscillator of a semiconductor laser element. This package is provided as a box-shaped, flattened module. This semiconductor laser device has a recess at a central portion of the principal plane of a metallic stem of the module. The recessed portion is sealed with a cap which is made of a metal plate. The sealed module is provided with a semiconductor laser element (or a laser chip) and a light receiving element which is made operative to detect the power or frequency of a laser beam emitted from the end face of the oscillator of the laser element or chip. In the recess, there also exists a conductor for applying an a.c. signal to the semiconductor laser element and a conductor for extracting an electric signal from the light receiving element. The aforementioned laser module for optical communications exhibits a sufficient function as a semiconductor laser device for optical communications. Despite this fact, however, this laser module is required, like other devices, to have higher performance characteristics.
At the present stage of development of optical communications, the information transmission rate is about 140 to 200 Mbits/sec, but is expected to be 560 Mbits/sec or several Gbits/sec in the future, in accordance with the expected drastic increase in the amount of information to be transmitted.